This invention relates in general to satellite-based communication systems, and specifically, to satellite-based mobile telecommunication systems.
Satellite communication systems are well known in the prior art. Examples of such systems are disclosed in U.S. Pat. No. 5,303,286 and other publications that are of record in said patent. In satellite communication systems, user terminals and gateways generally communicate with each other via one or more co-visible satellites (i.e. satellites xe2x80x9cseenxe2x80x9d by both the user terminals and the gateways). Some of the user terminals have broad beam antennas which illuminate much of the sky. The broad beam illumination contributes to interference with other user terminals using the covisible satellites. Furthermore, user terminals and gateways of the satellite communication system may communicate using a spread spectrum (SS) code division multiple access (CDMA) technique. The nature of communication using SS CDMA method is that the signal from a single user terminal is spread across the entire bandwidth of a given communication channel. Therefore, all user terminals communicating on a given communication channel may contribute to interference with another user terminal communicating on that channel. An increase in the number of user terminals on a given communication channel tends to increase overall interference, as does an increase in any individual user terminal""s transmit power when it is desired to boost the signal over the overall interference level of the channel.
It is a first object and advantage of this invention to provide a system and method to minimize total interference within a given channel of a satellite communication system.
It is a second object and advantage of this invention to provide a satellite communication system having the ability to assign communication channels to user terminals to achieve optimal performance of the satellite communication system.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention, wherein in accordance with a first method of the present invention, a method for maximizing capacity of a satellite communication system is provided. The method comprises the steps of finding a total interference in each frequency channel, calculating a predicted total interference from the addition of a first user terminal on each frequency channel, determining if the predicted total interference in a first channel is a minimum value, and allocating the first channel to the first user terminal. The total interference is found for each channel of a plurality of channels which subdivide a predetermined frequency band of a return link for at least two satellites. The predicted total interference from the addition of the first user terminal is calculated in each channel of the plurality of channels in the return link for each of the least two satellites. A determination of whether the predicted total interference is a minimum value in the first channel is made with respect to all predicted total interference values for the plurality of channels in the return link. The first channel is allocated to the first user terminal if the predicted total interference of the first channel is the minimum value.
In accordance with a second aspect of the present invention, a method is disclosed for assigning a frequency channel to a user terminal of a satellite communications system. The user terminal is assumed to be visible to at least two satellites. The method comprises the steps of identifying a location of the user terminal, determining if a first frequency channel of a plurality of frequency channels has a minimum total interference and, if yes, assigning the first frequency channel to the user terminal. If not, a next step determines if a second frequency channel has a total interference below a predetermined threshold and, if yes, assigns the second frequency channel to the user terminal. If this test fails, the method then determines if a third frequency channel has a total interference below the predetermined threshold for a first one of the two satellites, and a total interference above the predetermined threshold for a second one of the two satellites. If yes, the method determines if the first satellite is at a lower elevation angle than the second satellite, relative to the user terminal, and if yes, the method assigns the third channel to the user terminal, otherwise a fourth frequency channel is assigned for the return link of the user terminal. The location of the user terminal may be identified when the user terminal requests service. Determination of whether the first frequency channel has a minimum total interference is made for the return link of the user terminal to each one of the two satellites. The determination if the second frequency channel has a total interference below the predetermined threshold is also made for the return link of the user terminal to each satellite.